New MESSENGER Maps of Mercury’s Surface Chemistry Provide Clues to the Planet’s History

Two new papers from members of the MESSENGER Science Team provide global-scale maps of Mercury’s surface chemistry that reveal previously unrecognized geochemical terranes — large regions that have compositions distinct from their surroundings. The presence of these large terranes has important implications for the history of the planet.

These are the first global geochemical maps of Mercury, and the first maps of global extent for any planetary body acquired via the technique of X-ray fluorescence, by which X-rays emitted from the Sun’s atmosphere allow the planet’s surface composition to be examined. The global magnesium and aluminum maps were paired with less spatially complete maps of sulfur/silicon, calcium/silicon, and iron/silicon, as well as other MESSENGER datasets, to study the geochemical characteristics of Mercury’s surface and to investigate the evolution of the planet’s thin silicate shell.

The most obvious of Mercury’s geochemical terranes is a large feature, spanning more than 5 million square kilometers. This terrane “exhibits the highest observed magnesium/silicon, sulfur/silicon, and calcium/silicon ratios, as well as some of the lowest aluminum/silicon ratios on the planet’s surface,” writes Shoshana Weider, a planetary geologist and Visiting Scientist at the Carnegie Institution of Washington. Weider and colleagues suggest that this “high-magnesium region” could be the site of an ancient impact basin. By this interpretation, the distinctive chemical signature of the region reflects a substantial contribution from mantle material that was exposed during a large impact event.

A second paper, “Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements,” now available online in Icarus, presents the first maps of the absorption of low-energy (“thermal”) neutrons across Mercury’s surface. The data used in this second study were obtained with the GRS anti-coincidence shield, which is sensitive to neutron emissions from the surface of Mercury.

“From these maps we may infer the distribution of thermal-neutron-absorbing elements across the planet, including iron, chlorine, and sodium,” writes lead author Patrick Peplowski of The Johns Hopkins University Applied Physics Laboratory. “This information has been combined with other MESSENGER geochemical measurements, including the new XRS measurements, to identify and map four distinct geochemical terranes on Mercury.”

According to Peplowski, the results indicate that the smooth plains interior to the Caloris basin, Mercury’s largest well-preserved impact basin, have an elemental composition that is distinct from other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from the XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements.

“Earlier MESSENGER data have shown that Mercury’s surface was pervasively shaped by volcanic activity,” notes Peplowski. “The magmas erupted long ago were derived from the partial melting of Mercury’s mantle. The differences in composition that we are observing among geochemical terranes indicate that Mercury has a chemically heterogeneous mantle.”

“The consistency of the new XRS and GRS maps provides a new dimension to our view of Mercury’s surface,” Weider adds. “The terranes we observe had not previously been identified on the basis of spectral reflectance or geological mapping.”

“The crust we see on Mercury was largely formed more than three billion years ago,” says Carnegie’s Larry Nittler, Deputy Principal Investigator of the mission and co-author of both studies. “The remarkable chemical variability revealed by MESSENGER observations will provide critical constraints on future efforts to model and understand Mercury’s bulk composition and the ancient geological processes that shaped the planet’s mantle and crust.”

Early in its primary orbital mission, MESSENGER discovered thousands of peculiar depressions at a variety of longitudes and latitudes, ranging in size from tens of meters to several kilometers across and tens of meters deep.

“These features, given the name ‘hollows,’ were a major surprise, because while we had been thinking of Mercury as a relic — a planet that wasn’t really changing anymore — hollows appear to be younger than the planet’s freshest impact craters. This finding suggests that Mercury is a planet whose surface is still evolving," says MESSENGER Participating Scientist David Blewett, a geologist at The Johns Hopkins University Applied Physics Laboratory (APL).

The team has since deduced that the hollows form through loss of a component in the rocks that is susceptible to sublimation (or a similar process) when exposed to the harsh environment of the planet's surface. “High-resolution images obtained by the spacecraft at low altitudes are revealing striking details about these hollows, including their young ages, their depths, and the diversity of locations in which they are found.”

Mercury’s magnetic field, generated by a dynamo process in its outer core, has been in place far longer than previously known, a paper by MESSENGER Participating Scientist Catherine Johnson reports.... With MESSENGER orbiting Mercury closer than 100 kilometers from the planet’s surface, the spacecraft’s Magnetometer instrument that measures magnetic field strength and detection was able to resolve signals too small to be detected earlier at higher altitudes. The observed decrease in signal strength measured with changes in altitude from 15 to 80 kilometers confirms that the signals are due to the presence of magnetized crustal rocks, Johnson said.

Mercury is the only inner solar system body other than Earth that currently possesses a global magnetic field generated by a dynamo in a fluid metallic outer core. In Mercury, as in Earth, the outer core is molten iron.

Mercury’s magnetic field, generated by a dynamo process in its outer core, has been in place far longer than previously known, a paper by MESSENGER Participating Scientist Catherine Johnson reports.... With MESSENGER orbiting Mercury closer than 100 kilometers from the planet’s surface, the spacecraft’s Magnetometer instrument that measures magnetic field strength and detection was able to resolve signals too small to be detected earlier at higher altitudes. The observed decrease in signal strength measured with changes in altitude from 15 to 80 kilometers confirms that the signals are due to the presence of magnetized crustal rocks, Johnson said.

Mercury is the only inner solar system body other than Earth that currently possesses a global magnetic field generated by a dynamo in a fluid metallic outer core. In Mercury, as in Earth, the outer core is molten iron.

While researching electromagnetism, I found that Mercury is unique because it is the only rocky planet with a magnetic axis (N & S magnetic pole). Earth's moon, Venus, and Mars posses magnetic fields but lack a magnetic axis. The 4 gaseous planets posses a magnetic axis. The south magnetic axis of Earth, Mercury, and Uranus are located near their geographic north pole. Until last week, I always thought that Earth's geographic and geomagnetic north were both located in the arctic.

jtb wrote:While researching electromagnetism, I found that Mercury is unique because it is the only rocky planet with a magnetic axis (N & S magnetic pole). Earth's moon, Venus, and Mars posses magnetic fields but lack a magnetic axis. The 4 gaseous planets posses a magnetic axis. The south magnetic axis of Earth, Mercury, and Uranus are located near their geographic north pole. Until last week, I always thought that Earth's geographic and geomagnetic north were both located in the arctic.

jtb wrote:...The south magnetic axis of Earth, Mercury, and Uranus are located near their geographic north pole. ...

Well, dang, i never thought of that either...

From Wikipedia: "The convention in early compasses was to call the end of the needle pointing to the Earth's North Magnetic Pole the "north pole" (or "north-seeking pole") and the other end the "south pole". Because opposite poles attract, this definition means that the Earth's North Magnetic Pole is actually a magnetic south pole and the Earth's South Magnetic Pole is a magnetic north pole."

jtb wrote:...The south magnetic axis of Earth, Mercury, and Uranus are located near their geographic north pole. ...

Well, dang, i never thought of that either...

From Wikipedia: "The convention in early compasses was to call the end of the needle pointing to the Earth's North Magnetic Pole the "north pole" (or "north-seeking pole") and the other end the "south pole". Because opposite poles attract, this definition means that the Earth's North Magnetic Pole is actually a magnetic south pole and the Earth's South Magnetic Pole is a magnetic north pole."

Sorry, but I am unable to copy the table of data on this page(without it getting scrambled).If you scroll down a bit there is the table of INCLINATIONS.

HERE IS MY QUESTION: ( I am talking about the planet ORBITS. Also, THE ECLIPTIC is the orbit of earth)

The angles given for Mercury do not make sense. How can Mercury be inclined 7.01* to the Ecliptic and at the same time be inclined 3.38* to the suns equator. The earth( the ecliptic ) is inclined 7.155* to the suns equator. Thus, Mercury should be inclined to the suns equator either .145* or 14.165*; either the sum or the difference.These same or similar numbers are used on many other sites.The other planets numbers roughly add up to the correct values for planets between the ecliptic and the sun's equator.

Jack, I do believe that you are correct and someone made an error and everyone else faithfully copied this error. One would need to go to the library or an excellent orbital simulation that has the Sun's equator in it. Because I cannot find anything useful on the internet.

It is possible to draw a diagram where the north pole of the Sun is pointing in the direction which is closest to the Earth. The Earth at some point in it's orbit is in this position. Then the ecliptic is at 90 degrees to the paper this diagram is on. But the plane of the equator of the Sun is also at 90 degrees to the paper.

It is tempting to draw in Mercury at 7 degrees for the Mercury - Sun - Earth angle in the above diagram, but I see now that this is an error. Mercury should be drawn in at 3.77 degrees in this diagram and it is about 45 degrees further around the Sun that Mercury would be 7 degrees from the ecliptic.

So when it is stated that Mercury is 7 degrees from the ecliptic, that actually means at maximum it is 7 degrees from the ecliptic in only two positions in it's orbit around the Sun. In other positions it might be 2 degrees, for example, from the ecliptic, or even in the plane of the ecliptic.

Moses,Thanks for your help.I think I understand what you are saying although your first paragraph is not clear to me.

this is more clear.

So when it is stated that Mercury is 7 degrees from the ecliptic, that actually means at maximum it is 7 degrees from the ecliptic in only two positions in it's orbit around the Sun.

You are looking at the earth and Mercury at specific points in each respective orbit, and taking angles.I suggest that you don't go there.

The angles in question above are made by the conceptual planes representing the TOTAL ORBITS of each planet relative to the equatorial plane of the sun (as a reference plane). And another set of angles of each planets orbit plane relative to the orbit plane of the earth, called the ecliptic. as the reference.

The actual position of each body relative to either reference plane will change from Max plus degrees above the plane to zero, then to max negative degrees below the plane, etc. as it moves in orbit. I suggest we not talk about those angles for now.

Adding to the complexity, each planets max position above the solar eq. plane, as reference, will take place at a different location around the sun as referenced by the celestial sphere outside the solar system. This is an assumption on my part. I don't know what to call this or how to look it up.?

And then there is precession.Wiki:

The orbits of a planet around the Sun do not really follow an identical ellipse each time, but actually trace out a flower-petal shape because the major axis of each planet's elliptical orbit also precesses within its orbital plane, partly in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession or apsidal precession.

And

The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to the Sun. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun.[1]

The University where I live has an extensive astronomy library. I will try to get there and look for an answer to this Mercury orbit question.

Jack, please try this drawing exercise. Get paper and pen. Draw two circles, one representing the Sun and the other the Earth. Draw a line through the centre of the Sun at about 10 or 20 degrees from upright. This will represent the axis of spin of the Sun. Draw in a line perpendicular to this axis through the centre of the Sun. This will represent the equator of the Sun.

Now realise that there is only one position in the Solar System that one can view the Sun - Earth so that the north pole of the Sun is as close as it can get to Earth. Next visualise where the ecliptic plane is - use your hand to represent the ecliptic plane. It will come straight out of the paper at 90 degrees. Do the same visualisation of the plane of the equator of the Sun. It also is at 90 degrees to the paper. So far this should be clear if you have drawn the diagram.

So now we will attempt to draw another circle which will represent the position of Mercury. It is very tempting to draw this Mercury circle so that the Mercury - Sun - Earth angle is 7 degrees because the plane of Mercury's orbit is 7 degrees from the ecliptic. This is the error.

If you are in a spacecraft in the position that views the Sun - Earth as depicted in the diagram, which would be one day a year say, then it would probably be many years before one would find that Mercury is in a position between the Sun and the Earth. And when it finally was in position one would find that Mercury is 3.77 degrees from the plane of the Sun's equator, and so about 3.23 degrees from the ecliptic. To be 7 degrees from the ecliptic Mercury would be about 45 degrees further around the Sun.

If you are in a spacecraft in the position that views the Sun - Earth as depicted in the diagram, which would be one day a year say, then it would probably be many years before one would find that Mercury is in a position between the Sun and the Earth. And when it finally was in position one would find that Mercury is 3.77 degrees from the plane of the Sun's equator, and so about 3.23 degrees from the ecliptic. To be 7 degrees from the ecliptic Mercury would be about 45 degrees further around the Sun.

All this description throws me off. Each planet is in the solar equatorial plane twice in its orbit. Planet position in its orbit should be irrelevant to inclination angles of the orbital planes.

Now this:

Next visualise where the ecliptic plane is - use your hand to represent the ecliptic plane. It will come straight out of the paper at 90 degrees. Do the same visualisation of the plane of the equator of the Sun. It also is at 90 degrees to the paper. So far this should be clear if you have drawn the diagram.

I almost get this part: Two lines A) sun eq. plane and 7* rotated B)ecliptic. Both are 90* out from the page.Next: Where do you draw the next line for Mercury ?, and what is the angle to the page ?

My brain is stuck on two planes.

I FINALLY GET IT. THANKS, MOSES 1It was my own version of the three body orbit problem I guess.